The use of optical inspection methods to evaluate ion implants has been well known for some time. Successful measurements have been made with equipment in which an intensity modulated pump beam is used to periodically excite a small spot on the sample surface. The effects of the periodic excitation, which tend to generate thermal and/or plasma waves, are monitored with an optical probe beam. One such effect is periodic surface displacements which can be measured through interferometry or by monitoring periodic angular displacements of the probe beam. Another effect is periodic changes to the reflectivity of the sample which are monitored by measuring periodic changes in the power of a reflected probe beam. Further details of such systems can be found in U.S. Pat. Nos. 4,522,510; 4,636,088; and 4,854,710.
These systems were capable of adequately measuring a relatively wide range of ion implant dosage levels. In order to improve sensitivity to higher doses, various other approaches have been taken. In one approach, the steady state reflectivity of one or more single wavelength probe beams was measured and combined with the thermal wave data to reduce ambiguities. Such an approach is described in U.S. Pat. No. 5,074,669.
Additional efforts to increase the measurement capabilities of such systems included varying the distance between the pump and probe beam spots; varying the modulation frequency of the pump source; and combining the thermal wave data with other measured data such as from spectroscopic reflectometry or ellipsometry. Such efforts are described in U.S. Pat. No. 5,978,074 and copending U.S. patent application Ser. No. 09/499,974, filed Feb. 8, 2000. All of the above cited patents and patent applications are incorporated by reference.
The above described techniques do not function to measure ion concentration directly, rather, they measure the damage done to the crystal lattice structure by the implanted ions. Variations in dosage level produce different levels of damage which can be detected by the thermal wave measurements. Variations in the energy used to implant the ions also affects the extent of damage to the lattice. As the energy level is increased, the ions are driven deeper into the lattice and the damage is more extensive.
It would be desirable to develop a measurement method which could separate out the contributions of the dose and energy levels of the implants to the damage of the wafer. In this way, the process used to create the implants can be better controlled. Such a measurement would extremely useful in the fabrication of shallow junctions in semiconductors.
More specifically, in the effort to achieve further miniaturization of semiconductor devices, the junctions dimensions must be reduced, both in width and depth. According to the 1999 SIA international roadmap, the next technology node to be achieved in two years is characterized by a lateral channel length of 130 nm, which means that the vertical drain and source pn-junction depths have to be shallower than 100 nm. Low energy ion implantation (<5 keV) has been developed to achieve these ultra-shallow junction depths,
The need to create these shallow junctions requires unprecedented control of the ion implantation process. Any unexpected variations in either dosage level or energy of the implant can result in the failure of the circuit. Therefore, it would be highly desirable to adapt the prior measurement approaches to evaluate both dosage level (ion concentration) and the energy of the implants.
Research experiments have concentrated on using destructive methods such as secondary ion mass spectrometry (SIMS) transmission electron microscopy (TEM) and spreading resistance depth profiling. Some attempts for non-destructive analysis have been made with ion scattering and spectroscopic ellipsometry, while the non-destructive thermal wave methods have demonstrated low sensitivity for implants below 5 keV.
Most SIMS equipment have a physical limitation for accurate depth profiling of ultra-shallow junctions. A transient region down to 100 Å depth is typically formed at the oxygen bombarded surface due to ionization effects at the oxidized silicon surface. Special test samples are typically required with a silicon capping layer to avoid the surface effect. TEM imaging involves tedious cross-sectional sample preparation, but is generally considered the most accurate way to measure the crystalline damage depth. Spreading resistance depth profiling requires an electrical contact to be established to the wafer surface. Specialized probe conditioning and sample preparation are needed for reliable measurement of ultra-shallow junctions and currently only a few labs have succeeded in these analyses. The ion scattering methods are restricted to give the depth distribution of the displaced silicon atoms only and have been found to lack the sensitivity to detect defects at levels which are important in device operation. Spectroscopic ellipsometry has been used with simple 1–2 layer models with effective medium approach for layer mixing, which complicates the analyses as separate recipes are needed for high and low (<2.5 keV) ion implants.